1. Field of the Invention
The present invention relates to a circuit for generating two periodic signals having a controlled phase relationship and more particularly to circuitry which generates the two periodic signals with a phase relationship of from zero to 180 degrees, which phase is dependent on the amplitude of a d.c. control signal.
2. Description of the Prior Art
In many control systems an output parameter such as voltage, current or power is controlled by controlling the displacement angle between a primary waveform and a phase shifted version of the primary wave. The displacement angle may fluctuate anywhere from zero degrees to 180 degrees. Various techniques have been developed in the prior art for controlling this displacement angle. One such prior art technique is the use of a phase-locked loop.
The phase-locked loop system locks two oscillators into frequency synchronization and controls the phase angle between them by manipulating the phase detector parameters. The phase-locked loop system is, however, subject to problems. These problems arise as a result of instabilities in the control loop and difficulties in the operation of the capture and track modes of the phase-locked loop. When used as part of a larger control system these problems may make the larger system extremely difficult to stabilize.
The circuit of the present invention allows the phase angle between the main output and the phase shifted version thereof to vary in a controllable manner between zero degrees and 180 degrees without any of the stability problems associated with the previously described phase-locked loop system.